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            "BGP AS Path Prepending"

              "Examples on how to us BGP AS Path Prepending"

Author: David Bombal
CCIE, CCSI, CCDP, CCIP, CCNP, CCSP, CCVP

Network Diagram:

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AIM: Force ISPs to use line between R2 and R4 when traffic is sent to AS1
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R3 trace:
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R6 trace:
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We want to force the ISPs to use upper link - MED won't work as lost between AS 2 and AS 4 - Not happening now!

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R5 trace:
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We want to force the ISPs to use upper link - MED won't work as lost between AS 2 and AS 4 - Not happening now!

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Traffic Flow:
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Green = out of the AS

Red = into the AS

As can be seen, return traffic will sometimes use the bottom link - we want it to always use the top link.

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Config on R3:
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router bgp 1
no synchronization
bgp router-id 3.3.3.3
bgp cluster-id 151587081
network 3.3.3.3 mask 255.255.255.255
network 9.9.9.9 mask 255.255.255.255
network 10.1.3.0 mask 255.255.255.0
network 10.1.4.0 mask 255.255.255.0
neighbor 2.2.2.2 remote-as 1
neighbor 2.2.2.2 ebgp-multihop 255
neighbor 2.2.2.2 update-source Loopback0
neighbor 2.2.2.2 route-map setnexthop in
neighbor 5.5.5.5 remote-as 4
neighbor 5.5.5.5 ebgp-multihop 255
neighbor 5.5.5.5 update-source Loopback0
neighbor 5.5.5.5 send-community
neighbor 5.5.5.5 route-map setmetricout out       <==== Route map is applied to
                                                                                               neighbor R5
no auto-summary
!
ip classless
ip route 2.2.2.2 255.255.255.255 10.1.3.1
ip route 5.5.5.5 255.255.255.255 10.1.4.2
no ip http server
ip as-path access-list 1 permit ^2$
!
access-list 1 permit any
access-list 10 permit any
route-map setmetricout permit 10
match ip address 10
set metric 200
set as-path prepend 1 1 1 1       <==== Prepend the AS1 four times to make path
                                                                   longer for all routes
!
route-map setnexthop permit 10
set ip next-hop peer-address
!
route-map setweightin permit 10
match as-path 1
set ip next-hop peer-address
set weight 100

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Output on R5:
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As you can see above, the trace went via the upper link!! As expected!

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Output on R6:
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I hope that this has been beneficial to you.

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